I'm a new mother and my 7-week-old hasn't passed a stool in three days! Sometimes she goes a week without passing stool. When she does the stools are normal, but she's also gassy and cries a lot so I think something is wrong. Is this normal?

Yes, it is very normal (and expected) for babies around 6-8 weeks to slow down their frequency of pooping. Their intestines are maturing and they are more efficient in their digestion than the first several weeks of life. As long as the poop is soft when it comes out, it is fine if your baby poops just 1-2 times a week. Constipation, by definition, is not how often someone poops, but what it looks like when it comes out. If your baby is clearly uncomfortable, that is a different story, and you can try offering a few teaspoons of prune juice a day to keep things regular.